Presence of mind

Take a look at this marvelous answer:

A mechanic was removing the cylinder heads from the motor of a car when he spotted the famous heart surgeon in his shop, who was standing off to the side, waiting for the service manager to come to take a look at his car. The mechanic shouted across the garage,”Hello Doctor! Please come over here for a minute.”

The famous surgeon, a bit surprised, walked over to the mechanic. The mechanic straightened up, wiped his hands on a rag and asked argumentatively, “So doctor, look at this. I also open hearts, take valves out, grind ’em, put in new parts, and when I finish this will work as a new one. So how come you get the big money, when you and me is doing basically the same work? ” The doctor leaned over and whispered to the mechanic …..

What did he say ???

Guess
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He said : “Try to do it when the engine is running”.